Naturally occurring hydrotalcite is a mineral found in relatively small quantities in a limited number of geographical areas, principally, in Norway and in the Ural Mountains. Known deposits of the material total only about 2,000 to 3,000 tons worldwide. Natural hydrotalcite has a variable composition depending on the location of the source. Natural hydrotalcite is a hydrated magnesium, aluminum and carbonate-containing composition, which has been found to have the typical composition, represented alternatively as: 6MgO.Al.sub.2 O.sub.3.CO.sub.2.12H.sub.2 O and Mg.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O. Natural hydrotalcite deposits are generally found intermeshed with spinel and other materials due to the existence of non-equilibrium conditions during formation of the deposits. Other minerals such as penninite and muscovite, and heavy metals are also found in natural hydrotalcite deposits. There are no presently known techniques for separating these materials and purifying the natural hydrotalcite.
Synthetically produced hydrotalcite can be made to have the same composition as natural hydrotalcite, or, because of flexibility in the synthesis, it can be made to have a different composition by replacing the carbonate anion with other anions, such as phosphate ion.
A phosphate-modified synthetic hydrotalcite and a process for its synthesis is disclosed in U.S. Pat. No. 4,883,533 to Kosin et al. for "Modified Synthetic Hydrotalcite." The hydrotalcite disclosed there is of the general formula xMgO.Al.sub.2 O.sub.3 yPO.sub.4.zH.sub.2 O wherein x, y and z are integers representing the number of moles of each component, and wherein x is from 2.5 to 4.5, y is from 1.0 to 2.0 and z is from 7.0 to 10.0, with the hydrotalcite having a platelet particle morphology and an average particle size of less than about 1 micron.
U.S. Pat. No. 3,539,306 to Kumura et al. for "Process for the Preparation of Hydrotalcite" discloses a process for preparing hydrotalcite which involves mixing an aluminum-containing compound with a magnesium-containing compound in an aqueous medium in the presence of carbonate ion at a pH of at least 8.
U.S. Pat. No. 4,656,156 to Misra for "Absorbent and Substrate Products and Method of Producing Same" discloses a process for producing synthetic hydrotalcite, which is subsequently mixed with activated alumina to form absorbent and substrate compositions. The process for producing synthetic hydrotalcite involves the steps of forming activated magnesia by heating a magnesium compound to a temperature of from about 500.degree. to 900.degree. C., then adding the activated magnesia to an aqueous solution containing aluminate, carbonate and hydroxyl ions, such as industrial Bayer process liquor, and agitating at a temperature of about 80.degree.-100.degree. C. for 20-120 minutes to form a low density, high porosity hydrotalcite.
A method for producing hydrotalcite from activated magnesia by reacting it with an aqueous solution containing aluminate, carbonate and hydroxyl ions is disclosed in U.S. Pat. No. 4,904,457 to Misra for "Synthetic Hydrotalcite". The method includes a first step of heating magnesium carbonate or magnesium hydroxide to a temperature between 500.degree.-900.degree. C. to form activated magnesia. The hydrotalcite produced by this process is the same as that disclosed in U.S. Pat. No. 4,656,156.
Hydrotalcite compositions containing pillaring organic, inorganic and mixed organic/inorganic anions are disclosed in U.S. Pat. No. 4,774,212 to Drezdon for "Pillared Hydrotalcites". The compositions are anionic magnesium aluminum hydrotalcite clays having large inorganic and/or organic anions located interstitially between positively charged layers of metal hydroxides. The compositions are of the formula EQU [Mg.sub.2x Al.sub.2 (OH).sub.4x+4 ]Y.sub.2/n.sup.n-.ZH.sub.2 O
where Y is a large organic anion selected from the group consisting of lauryl sulfate, p-toluenesulfonate, terephthalate, 2,5-dihydroxy-1,4-benzenedisulfonate, and 1,5 naphthalenedisulfonate or where it is an anionic polyoxometalate of vanadium, tungsten or molybdenum. In the above cases, x is from 1.5 to 2.5, n is 1 or 2 and Z is from 0 to 3, except that when Y is polyoxometalate, n is 6.
Synthetic hydrotalcite produced according to all previous methods has been known to have either a plate-like or a needle-like morphology and is unaggregated or forms irregularly shaped aggregates of broad aggregate size distribution. It has heretofore been unknown in the art to produce an aggregated hydrotalcite having a substantially uniform shape and an aggregate particle size which is uniform, and which can be controlled over a wide range of sizes, having a narrow aggregate particle size distribution.
We have discovered that, surprisingly, aggregates of synthetic platy hydrotalcite, having substantially uniformly spheroidal shapes, with relatively narrow aggregate size distribution, low specific density, high surface area, and high porosity can be made by carefully controlling a number of reaction parameters in the synthesis of the hydrotalcite. The ability to synthesize such a form of hydrotalcite makes the material of the present invention suitable for a large number of diverse applications for which needle-like hydrotalcite, unaggregated platy hydrotalcite, and irregularly shaped aggregated hydrotalcite are deemed unsuitable, or are non-optimal.